Dr. Michael Hofmann
Applied Stratigraphy Research Group
Department of Geosciences
The University of Montana

The Piceance Basin in northwest Colorado is a major natural gas producing basin in the State of Colorado. Despite recent success in developing shale gas resources in the basin, the main gas bearing unit in the basin remain the tight gas sandstones of the Upper Cretaceous lower Williams Fork Formation.

The lower Williams Fork Formation in the Piceance Basin is a very low (~20%) net-to-gross succession, composed of isolated channel sandstone bodies encased in overbank mudrock interpreted as meandering fluvial systems of varying sinuosity within a coastal-plain, marginal marine setting.

Analysis of sand-body distribution reveals that fluvial channel sands are not randomly distributed but are predictable in their spatial and stratigraphic position that correlate to "sweet spots". Sand bodies are organized in distinct intervals of clustered channel belts, each ~400 ft (~122 m) thick, topped by a thin coal layer. The resulting channel-belt clusters are compensationally stacked. This channel organization occurs on different time scales, channel clustering is the dominant form of channel organization over a shorter period (channel and channel-belt scale 10s ky to 100s ky), compensational stacking occurred over a much longer time scale (channel cluster belts, ~ 400 ky).

The primary control on the cluster formation is autogenic channel avulsion during an overall aggradational phase. Short-lived changes in basin accommodation caused by either changes in tectonic subsidence or high-frequency eustatic changes due to Milankovitch-band orbital forcing at the end of each cluster interval result in a distinct increase in channel thickness and coal formation towards the end of each cycle. On the longest time scale (~ 1 to 1.5 My), which encompasses the entire lower Williams Fork, changes in channel geometries and sand-body thickness seem to be controlled mainly by long-term changes in eustasy, and autogenic processes are of lesser importance.